1. Field of the Invention
This invention relates to a cyclic process for direct conversion of low concentrations of hydrogen sulfide (H2S) in gas mixtures to elemental sulfur by catalytic oxidation over an activated carbon catalyst, sorption of the sulfur product by the said carbon and subsequent desorption and recovery of the sulfur during regeneration of the catalyst.
2. Prior Art
Current practice in the sour natural gas processing industry is to remove the acid components (H2S and CO2) from the natural gas by a sweetening process. The removed hydrogen sulfide, if present in small quantities, is incinerated to sulfur dioxide (SO2) and vented to the atmosphere provided that the amount of released SO2 is acceptable according to regulations with respect to the environment. If the quantity of H2S removed by a sweetening process is sufficiently large, it is generally fed to a Claus plant and recovered as elemental sulfur.
Recent more stringent regulations in certain jurisdictions concerning the release of sulfur as SO2 to the atmosphere have made the sour gas processing industries aware that they will be required to reduce sulfur-containing emissions to the environment substantially. With the increasing demand for elemental sulfur and the need to meet the existing environmental regulations, considerable attention has been given to the development of inexpensive and effective methods for the recovery of elemental sulfur from natural gases containing H2S.
It has been known for some time that hydrogen sulfide in natural gas or other gases can be oxidized in the presence of various catalysts to sulfur dioxide or sulfur. Examples of some of these processes may be found in the patent literature.
In Canadian Patent 1,172,428 by R. F. Jagodzinski and R. K. Kerr issued on Aug. 14, 1984, a process is disclosed whereby hydrogen sulfide in sour gas is reacted with oxygen at pressures greater than 5 atmospheres over an activated alumina or a vanadium pentoxide catalyst. The catalyst is continuously soaked and submerged in liquid sulfur in a reactor at temperatures between 250 and 550xc2x0 C. Elemental sulfur is produced along with a substantial fraction of SO2. The unreacted H2S from this first reactor is then reacted with the produced SO2 to produce elemental sulfur and water in a second reactor.
In Canadian Patent 1,063,321 by W. H. Powlesland and J. W. Smith issued on Oct. 2, 1979, a process is disclosed whereby H2S from a xe2x80x9cfouled gasxe2x80x9d is removed by passing the gas through hydrated hematite (Fe2O3) pellets in a chamber, thereby producing water and forming elemental sulfur which coats the pellets. Pellets are continuously withdrawn from the bottom of the chamber to a tumbler where continuous tumbling of the pellets abrades the elemental sulfur from their surface. The sulfur is recovered and the abraded pellets are then continuously returned to the top of the chamber. The process is complex and the composition of the product stream is not given. It is stated that during low temperature regeneration of the product Fe2S3 by oxidation, the possibility of SO2 production is high because of the high temperature rise in the following reaction (1) which in turn can initiate reaction (2)
Fe2S3+1.5O2=Fe2O3+3S+144 kcal.xe2x80x83xe2x80x83(1)
Fe2S3+4.5O2=Fe2O3+3SO2+347 kcal.xe2x80x83xe2x80x83(2)
It is advised in the patent that reaction (2) be avoided if possible because of SO2 production.
In Canadian Patent 722,113 issued on Nov. 23, 1965, E. E. Baker and W. A. Duncan describe a process in which hydrogen sulfide in natural gas is oxidized in a bed of molecular sieve (crystalline zeolite) pellets having an apparent pore size of at least 4.6 Angstrom units (AU) at temperatures below 150xc2x0 F. and at a xe2x80x9cfirst higher pressuresxe2x80x9d (100-1000 psig) thereby adsorbing the hydrogen sulfide. A hydrogen sulfide-depleted natural gas stream is discharged from the first bed. The pressure in the first bed is then reduced to a xe2x80x9csecond lower pressurexe2x80x9d (50 psig) at which hydrogen sulfide and other gases are desorbed. The released gases are then adsorbed in a second molecular sieve bed of crystalline zeolite again having an apparent pore size of at least 4.6 A.U. at temperatures in the range of 350xc2x0-750xc2x0 F. The sorption is conducted in the presence of free oxygen so as to produce and recover elemental sulfur. This is a two-step process. There is no mention of SO2 production.
In Canadian Patent 1,117,276 by K. D. Henning et al. issued on Feb. 2, 2002, a process is disclosed for elimination of sulfur compounds, in particular hydrogen sulfide, from gases containing the same, by reaction with oxygen and/or SO2 in the presence of activated carbon at elevated temperatures to produce elemental sulfur. The process is performed at temperatures between 120 and 240xc2x0 C. and at pressures ranging from 1 to 50 bars and with O2/H2S molar ratios from 1.53 to 2.2 (i.e. 3.06 to 4.4 times the stoichiometric ratio). A two-step process is necessary if the H2S content in the feed gas exceeds 1318 ppm (i.e. 2 g H2S per m3 of feed gas). The regeneration of the carbon in the first adsorber is less frequent than that in the second adsorber because of its autoregeneration. When regeneration is required, it is carried out with a hot inert gas. The preferred particle size of the activated carbon is 3 to 6 mm. In the first adsorber the activated carbon has a medium pore radius between 7-12 A.U. while in the second adsorber it is 5-8 A.U. There is no mention of the effect of pressure on H2S conversion and SO2 production.
The disadvantages of the above-mentioned processes are that they are complicated and that some of them produce substantial amounts of SO2. None, except the last one, uses activated carbon as a catalyst during the catalytic oxidation of H2S. Although Patent 1,117,276 describes a process which is similar to the process being disclosed herein, it fails to recognize the positive effect of elevated pressure operation to achieve (a) high hydrogen sulfide conversions to elemental sulfur and (b) reduced SO2 production. In addition, it has been found that by use of lower O2/H2S ratios than the above patent describes, lower SO2 production can be achieved at high H2S conversion levels.
The above mentioned patents fail to recognize the deleterious effects of having traces of heavy hydrocarbons in the feed gas. Unless these components are removed by means of cryogenic equipment or a guard bed, the overall life of the catalyst will be reduced, the time between regenerations of the catalyst will be shortened and the quality of the product sulfur will deteriorate.
Canadian Patent 1,117,276 fails to recognize the value of operation at pressures beyond the range specified (1 to 50 bars), in terms of being able to use lower O2/H2S molar ratios which favor lower SO2 production, in terms of more effective utilization of the activated carbon catalyst between regenerations and in terms of reduced energy requirements in processing the gas ready for delivery to a pipeline for sale at pressures of, for example, 65 to 70 bars. Furthermore, the use of higher O2/H2S ratios than are required leaves more unconverted oxygen and the associated nitrogen in the gas causing dilution and a lower calorific value of the product.
The objectives of the present invention are to provide a simple and efficient process to oxidize H2S in gas mixtures catalytically in the presence of air and an activated carbon catalyst so as to produce substantially pure elemental sulfur and, simultaneously, to reduce the production of SO2 to acceptable levels so that the product gas can be fed directly into pipelines ready for use by consumers in the case of natural gas or it can be burned, flared or otherwise vented to the atmosphere. According to this invention:
Sour natural gas or other gas mixtures containing low concentrations (preferably in the range from 4 ppm to 5 mol %) of H2S is mixed with air/oxygen at (O2/H2S ratios of 1.0 to 3.0 but preferably 1.1 to 2.0 times the stoichiometric requirement) and passed through a reactor containing an activated carbon catalyst at temperatures between 130 and 220xc2x0 C. but preferably between 150 and 200xc2x0 C. with residence times from 1 to 90 seconds and at pressures ranging from 100 up to 7000 kPa. The particle size of the catalyst used was 2.38xc3x970.841 mm. It had a mean pore radius of 2.9 nm and a total pore volume of 1.0 cm3/g. Hydrogen sulfide in the gas mixture is oxidized to elemental sulfur and water with very small SO2 production according to the following reactions                                                                         H                2                            ⁢              S                        +                                          1                2                            ⁢                              O                2                                              →                      S            +                                          H                2                            ⁢              O                                      ⁢                  xe2x80x83                                    (        3        )                                          S          +                      O            2                          →                  S          ⁢                      xe2x80x83                    ⁢                      O            2                                              (        4        )            
It has been discovered that the higher the operating pressure in the reactor, the lower is the SO2 production. This is unexpected since Reaction 4 should be favored by higher pressure.
FIG. 1 shows the arrangement of the apparatus used to conduct the operations at pressures up to 640 kPa. Prior to entering the reactor, air and hydrogen sulfide are mixed so that the desired O2/H2S ratio is achieved. Initially, most of the produced sulfur is deposited in the micropores of the activated carbon catalyst. Gradually the catalyst loses its activity due to increased sulfur loading of the micropores. FIG. 2 provides results of experiments conducted at various pressures up to 640 kPa. It shows that with increased operating pressures in the reactor, virtually complete conversion of the hydrogen sulfide can be maintained for a longer period than is possible at lower pressures and with a substantial reduction in the SO2 production.
In view of the advantages observed from operation at higher pressures, further experiments were conducted at 5600 kPa using mass flowmeters to monitor the gas flows and a back pressure regulator to control the system pressures as can be seen in FIG. 3. The results of this experiment are given in FIG. 4. The results further confirm the advantages of elevated pressure operation in terms of high H2S conversion for longer periods accompanied by reduced SO2 production.
FIG. 5 shows the reduction in the vapor pressure of deposited sulfur due to the capillarity effect in the micropores of the catalyst. We have discovered that when these pores are filled, the vapor pressure of sulfur increases so that there is a greater tendency for Reaction 4 to proceed producing SO2. It is recognized that by increasing the reaction operating pressure the partial pressure of sulfur vapor in the reactor is reduced and that this decreases the rate of SO2 production. The catalyst may be loaded with sulfur up to 80-150% of its mass before its activity decreases significantly so that it requires regeneration. Operating conditions in the reactor are maintained so that virtually complete conversion of the H2S in the sour gas stream is achieved. The allowable sulfur loading on the catalyst will depend on the catalyst itself and the operating parameters employed such as space velocity, residence time, temperature and pressure in the reactor.